1. Field of the Invention
This invention relates to a rotation transmitter using balls, and more particularly to a rotation transmitter using balls comprising a worm gear and a worm wheel meshed with the worm gear via a large number of metal balls, wherein the worm gear is provided with a ball circulation path constituted by a helical race formed on, and a circulation passage formed to pass through, the interior of, the worm gear.
2. Prior Art Statement
Various rotation transmitters constituted of a worm gear and worm wheel meshed with the worm gear via a large number of metal balls have been proposed. Some specific examples can be found, for instance, in U.S. Pat. Nos. 3,468,179 and 4,951,519, GB Patent No. 2,164,719, French Patent No. 1,261,122 and Japanese Patent Publications Nos. 38(1963)-22759 and 48(73)-31172.
A typical example of the prior art in this field is shown in FIGS. 12, 13 and 14, which correspond to the disclosure of Japanese Patent Publication No. 38(1963)-22759. As shown, the disclosed rotation transmitter comprises a wheel gear 8, an hourglass-shaped worm gear 1 meshed with the wheel gear 8 via metal balls 5 and covered by an outer guide 7, and a pair of collars 4 provided at opposite ends of the worm gear 1. The ball circulation path of the hourglass-shaped worm gear 1 comprises a ball race 2 on the exterior surface of the worm gear 1 and a ball circulation passage 3 passing along the axis of worm gear 1. As shown in FIG. 14, a ball loading hole 19 is formed in each collar 4 at the junction between the ball race 2 and the ball circulation passage 3 and, after the metal balls have been inserted, the ball loading hole 19 is stopped with a plug 20.
As shown in FIG. 13, each collar 4 is formed with a fan-shaped projection 21 having a guide passage 22. The guide passage 22 is formed by curving the corresponding end of the axial ball circulation passage 3 outwardly and then gradually bending it in the tangential direction until it merges with the ball race 2 on the outer surface of the hourglass-shaped worm gear 1.
The ball circulation path is therefore constituted by the combination of the ball race 2 on the outer surface of the worm gear 1, the guide passage 22 formed in the end faces of the worm gear 1 and the collars 4, and the ball circulation passage 3 passing along the axis of the worm gear 1.
On the other hand, Japanese Patent Publication No. 48(1973)-31172 discloses a rotation transmitter using balls in which the ball circulation path is constituted by using curved guide tubes to connect a ball circulation passage formed inside an hourglass-shaped worm gear along axis thereof with a helical ball race formed on the outer surface of the hourglass-shaped worm gear.
Moreover, while the ball race 2 on the outer surface of the hourglass-shaped worm gear is formed as a gothic groove in order to prevent skew, as shown in FIG. 12, the conventional practice has been to define the position of straight lines extending radially outward from the center 16 of the circle defined by the profile line 15 in the axial direction of the worm gear 1 as radial reference lines 17 and to form the helical ball race 2 by machining from this direction.
Therefore, as shown in FIG. 15, the centermost radial reference line 17 and the axis 12 of the worm gear 1 meet to form left and right angles R which are substantially equal (i.e. right angles). However, with increasing distance from the center and with decreasing diameter of the circle defined by the profile line 15 of the worm gear 1, the angle 8 that the radial reference lines 17 form with the axis 12 becomes increasingly acute and the radial reference lines 17 shift toward the ends of the worm gear 1. As a result, the positions of the radial reference lines 17 become the apexes P of the gothic groove, and the ball race 2, whose standard machining position is the reference line 18 passing through the apexes P, comes to differ increasingly in sectional configuration from that at the center region with increasing proximity to the ends of the worm gear 1 and thus assumes a sectional configuration differing from that of a true gothic groove.
In a conventional rotation transmitter using balls such as that exemplified by Japanese Patent Publication No. 38(1963)-22759, it is extremely difficult to precision-machine, fabricate and adjust the ball circulation path so that the moving balls do not become clogged therein. Moreover, the guide passage formed in the fan-shaped projections of the collars for communicating the ball circulation path along the axis of the hourglass-shaped worm gear with the ball race on the outer surface thereof curve over the short distance to the axis and the end portions thereof connect with the ball circulation path by passing perpendicularly with respect to the ends of the worm gear. As a result, it cannot be expected that the balls will be able to flow smoothly, even if the diameter of the collars is made large.
The ball circulation path disclosed in Japanese Patent Publication No 48(1973)-31172 in which the ball circulation passage and the ball race are connected by guide tubes is an improvement on the ball circulation path disclosed in Japanese Patent Publication No. 38(1963)-22759 and it does in fact reduce the number of components and make fabrication simpler. Nonetheless it still involves a number of problems. Specifically, in providing guide tubes capable of enabling smooth passage of the balls, it is extremely difficult to impart curvature to the guide tubes and to precision-machine the connecting portions thereof in such a way as to maintain the inside diameter thereof within a close tolerance throughout the length thereof, minimize friction and reduce noise.
Moreover, the ball race formed on the outer surface of the conventional hourglass-shaped worm gear is formed as a helical gothic groove by machining in the direction of reference lines that are straight radial lines extending toward the outer surface of the worm gear from the center of the circle defined by the profile line in the axial direction of the worm gear. Since the sectional shape of the ball race therefore differs between the center and end regions of the hourglass-shaped worm gear, the skew preventing effect of the gothic groove is reduced and the contact between the groove and the balls becomes uneven at some positions, leading to an increase in rolling friction and noise. In particular, since the machining has been conducted by rotating the cutting tool in the radial direction, it has been difficult to realize high-precision machining.